GreenBuilding.com

Homes That Produce The Energy They Use

Green-built houses aren’t necessarily the same as net zero energy homes. The simple definition is that a net zero energy building produces as much energy as it uses on an annual basis. This includes energy for heating, cooling, and all the devices that plug into the wall. Net zero houses are typically connected to a local electric utility. They use the grid for storing excess electricity generated by photovoltaic panels or a wind turbine, banking electricity at time of plenty and drawing on the surplus when production falls.

A house in a cold climate might need more energy than it makes during the winter but then makes up for it in summer when demand is lower and photovoltaic system is running at full tilt. The opposite may be true in the south, where high humidity in the summer requires more electricity for air-conditioning during peak months. But on average, zero energy houses produce enough energy to offset the high-load months.

Paths to Zero Energy Homes

Grid-tied homes & net metering

Most grid-tied homes are built where local utility offers net metering. That means the utility will buy electricity at the same price it charges, but usually only until the net is zero. If houses produce more than that, the utility may buy it back. If so, it is often at the wholesale price, which can be as little as 1 or 2 cents per kilowatt hour when the retail price is 10 or 12 cents per kilowatt hour. That makes those excess electrons produced very expensive.

Electricity on a budget

Off-the-grid houses must provide all the electrical energy its occupants need, summer and winter. Other than relatively small battery banks, there is no place to store energy. The house is truly self-sufficient. For decades, a handful of builders around the country have experimented with off-the-grid approaches in different climates. They have taken very diverse paths to get to the end goal. Most often what makes the house self-reliant are changes in life-style for the families that live in them. Electricity goes on a budget. There is a fixed amount of energy available for any given day. If someone wants to take a hot shower, it has to be on a day with plenty of sun. If you want toast in the morning, maybe you can’t use the hair dryer.

Paying for embodied energy

Houses also can be designed to produce enough energy to offset the embodied energy in all the building materials plus the energy required to build the house. This means the house must produce more energy than it uses on a yearly basis. Roughly 8% of a home’s energy use is embodied energy from producing and transporting the building materials used in its construction. This is sometimes called regenerative architecture, and it has a deep ethical vein running through it.

Carbon-neutral homes

Carbon-neutral homes use a different metric to determine how to get to zero. More than just zero energy, it must be zero carbon emissions all the way back to the power plant or manufacturing facility that made the building products in the first place. On average, getting electricity from a power plant to a house is at best 30% efficient. From a carbon-neutral standpoint, the electricity used from the grid has to be repaid with three times more site-generated electricity to break even. The same holds true for building materials. If the marble tile in the foyer is from Italy, the energy produced at the house has to be sufficient to make up for the embodied energy from extraction and transportation of the marble. The utility buy-back policy dictates the financial context for this approach. Adherents to carbon-neutral houses are insistent on using only local materials and simple solutions to getting to zero energy. The more complex the house, the more diverse the sources of the materials and the more energy needs to be produced.

A house in a cold climate might need more energy than it makes during the winter but then makes up for it in summer when demand is lower and photovoltaic system is running at full tilt. The opposite may be true in the south, where high humidity in the summer requires more electricity for air-conditioning during peak months. But on average, zero energy houses produce enough energy to offset the high-load months.

Paths to Zero Energy Homes

Grid-tied homes & net metering

Most grid-tied homes are built where local utility offers net metering. That means the utility will buy electricity at the same price it charges, but usually only until the net is zero. If houses produce more than that, the utility may buy it back. If so, it is often at the wholesale price, which can be as little as 1 or 2 cents per kilowatt hour when the retail price is 10 or 12 cents per kilowatt hour. That makes those excess electrons produced very expensive.

Electricity on a budget

Off-the-grid houses must provide all the electrical energy its occupants need, summer and winter. Other than relatively small battery banks, there is no place to store energy. The house is truly self-sufficient. For decades, a handful of builders around the country have experimented with off-the-grid approaches in different climates. They have taken very diverse paths to get to the end goal. Most often what makes the house self-reliant are changes in life-style for the families that live in them. Electricity goes on a budget. There is a fixed amount of energy available for any given day. If someone wants to take a hot shower, it has to be on a day with plenty of sun. If you want toast in the morning, maybe you can’t use the hair dryer.

Paying for embodied energy

Houses also can be designed to produce enough energy to offset the embodied energy in all the building materials plus the energy required to build the house. This means the house must produce more energy than it uses on a yearly basis. Roughly 8% of a home’s energy use is embodied energy from producing and transporting the building materials used in its construction. This is sometimes called regenerative architecture, and it has a deep ethical vein running through it.

Carbon-neutral homes

Carbon-neutral homes use a different metric to determine how to get to zero. More than just zero energy, it must be zero carbon emissions all the way back to the power plant or manufacturing facility that made the building products in the first place. On average, getting electricity from a power plant to a house is at best 30% efficient. From a carbon-neutral standpoint, the electricity used from the grid has to be repaid with three times more site-generated electricity to break even. The same holds true for building materials. If the marble tile in the foyer is from Italy, the energy produced at the house has to be sufficient to make up for the embodied energy from extraction and transportation of the marble. The utility buy-back policy dictates the financial context for this approach. Adherents to carbon-neutral houses are insistent on using only local materials and simple solutions to getting to zero energy. The more complex the house, the more diverse the sources of the materials and the more energy needs to be produced.

Renewable Energy & Net Zero Homes

Photovoltaic panels that generate electricity from sunlight are typically a key ingredient in energy self-sufficiency. Wind turbines sized for residential use are another option, but in either case, the relatively high cost of renewable energy puts a premium on reducing heating and cooling loads.